Resistors, capacitors and inductances required in conventional filters or sensors may be replaced since the SAW device is capable of processing signals with the design of surface electrodes. The SAW device has the advantages of high performance, small size, low cost, and repetitive manufacturing. Therefore, the SAW device is important both in electronic industry and communication systems, especially in cellular phones which require high performance and small size.
Take SAW filter as an example, as shown in FIG. 1A, the piezoelectric substrate 10, such as quartz, Lithium Niobate or Lithium Tantalate, is coated with designed inter-digital transducers (hereafter referred to IDTs) 4 and 5 having interval D. After receiving high frequency signals 1, the IDT 4 will transform the received signals to surface acoustic waves 2. Then, the surface acoustic waves 2 are passed to IDT 5 through the piezoelectric substrate 10 and outputted after transformed to high frequency signals 3. In the aforementioned IDTs 4 and 5, each interval space between fingers is D, but these intervals may be changed or modified with different dimension according to the various demands or application. Hence, the input high frequency signals having the same resonance frequency with IDT 4 can be transformed to surface acoustic waves 2, and the surface acoustic waves 2 having the same resonance frequency with IDT 5 can be transformed to high frequency signals 3 efficiently. In above transformation, it is imperative to get rid of the unlimited signals and noises but let go signals with certain wavelengths (wavelength selection), to filter signals.
In the process of SAW device manufacturing, such as the procedure of forming IDTs, the SAW device has to bear some temperature rise and fall treatment such as the thermal cycle for curing photo-resistance. For the piezoelectric substrate is made of thermoelectric materials, the change in temperature would produce ionization electrons accumulated on the SAW device so as to generate electrostatic charges. The electrostatic discharge will damage the SAW device as well as alter their characteristics. In view of this, as shown in FIG. 1B, the Japan Patent No. 6-224682 discloses that not only IDTs 14a, 15a and reflection portions 14b, 15b, but also the sacrificial electrodes 16a, 17a for destroying the electrostatic charges are formed on the input electrode 12 and output electrode 13 of the piezoelectric substrate 11. Because of the sacrificial electrodes, the electrostatic discharging would take place on the sacrificial electrodes rather than IDTs 14a and 15a. 
Although aforementioned structure makes the electrostatic discharging occur on the sacrificial electrodes, however, it is not easy to control the discharging process, sometimes excessive electrostatic discharge would still destroy the sacrificial electrodes, thereby leading the sacrificial electrodes fail to perform the protection function. Hence, if there are more electrostatic charges, they may discharge on the IDTs 14a, 15a and the characteristics and functions of them would be altered and destroyed. Concerning the excessive electrostatic discharge, the other disadvantage is that the sacrificial electrodes will melt to contact each other to cause short circuit. Accordingly, Japan Patent No. 8-321739 improves forgoing structure. Resistant films 16 and 17 are attached on the sacrificial electrodes 16a and 17a to increase their resistance against electrostatic discharge, as shown in FIG. 1C. The IDTs can be reused to protect the SAW device from electrostatic discharge. However, the structures both require higher specific surface area to attain better resistance against electrostatic discharge. Besides, the path of electrostatic discharge is not controllable, and the sacrificial electrodes are those with smaller digital intervals. The oscillation thereof would influence the characteristics of the SAW device, so the sacrificial electrodes 16a, 17a must be disposed perpendicular to the IDTs 14a, 15a, respectively.
Japan Patent No. 5-121993 discloses another SAW device protection structure, as shown in FIG. 1D. Generally, the horizontal fingers 25, 27 are formed on the front end of the middle between the fingers 24a, 24b and 26a, 26b of the IDT 22 (23) to prevent the ESD break. This structure will increase the capacity of electrostatic charges and then effect the oscillation of the SAW device. Consequently, the horizontal fingers 25, 27 are moved afterward to avoid foregoing problem. Nevertheless, with this structure, it is easy to occur electrostatic discharging to damage SAW device in the position designated P between two digital transducers. Another improved structure is shown in FIG. 1E, the horizontal fingers 25, 27 are extended to form horizontal fingers 28 and adjacent fingers are connected to avoid such a problem. In this way, the properties of frequency response will be altered. Besides, the scope of usage is limited since the energy loss increases during the filtering process.
U.S. Pat. No. 6,034,578 discloses a SAW device with discharge electrodes electrically independent from the IDTs. As shown in FIG. 1F, IDTs 32, input wire bonding pad 33, output wire bonding pad 34, ground wire bonding pads 35, common electrodes 36, and thin film electrodes 37a, 37b, 37c and 37d are provided on the piezoelectric substrate 31. The thin film electrodes 37a, 37b, 37c and 37d are not connected to any aforementioned elements and electrically independent from them. In addition, the spaces S1 between the thin film electrodes 37a and 37b as well as between 37c and 37d are smaller than the spaces S2 and S3 between IDTs 32. The generation of electric charges due to spontaneous polarization is not uniform, and the quantity of charge generation is largest in the dicing margin portion. Since the space S1 is set to be sufficiently narrow, the static electricity is discharged between the thin film electrodes 37a and 37b or between 37c and 37d selectively. Accordingly, there is no fear that the IDTs 2 are subject to ESD breaking. However, this structure requires additional regions to dispose thin film electrodes 37a-37d so that the size thereof increases. Besides, there exist electrostatic charges in the inner region of thin film electrodes 37a-37d, too. These charges would attach to the elements such as IDTs 32. Since IDTs 32 are electrically independent from thin film electrodes 37a-37d, these electrostatic charges cannot be transferred to thin film electrodes 37a-37d. Hence, IDTs may suffer electrostatic breaking.
FIG. 1G shows a SAW device with the protection against electrostatic breaking according to U.S. Pat. No. 6,486,752. The digital transducers 41, 42, and 43 forms first IDTs 61, and the digital transducers 44, 45 and 46 forms second IDTs 62. The IDTs 61 and reflectors 63, 64 and 65 are combined with the series connected resonator, and the IDTs 62 as well as reflectors 67, 68, 69 and 70 are combined with the parallel resonator. The digital transducers 42 and 45 respectively connect to dicing line 55 via connection patterns 91 and 92. Digital transducers 41 and 44, 43, 46 connect to dicing line 55 via connection patterns 81, 84, 86, respectively. Reflectors 65, 66, 67 and 68 connect to dicing line 55 via connection pattern 86, and reflectors 63, 64, 69 and 70 connect to dicing line 55 via connection pattern 86, 82, 83, 85, respectively. Accordingly, all devices connect with dicing line 55 to discharge electrostatic charges thereon to avoid electrostatic break. But, the dicing line will be cut off so that the electrostatic protection vanishes. It is unavoidable to suffer electrostatic break for package process.